Wireless communication devices, which may typically be portable phones, may have an arrangement in which an antenna receives and sends an unbalanced signal, while an amplifier inputs and outputs balanced signals. An acoustic wave device is provided as a filter between the antenna and the amplifier may be equipped with a function of converting electrical signals that are balanced about ground to signals that are unbalanced and vice versa, that is, the function of balun.
FIGS. 1A and 1B are plan views of related Surface Acoustic Wave (SAW) devices.
Referring to FIG. 1A, the device has a piezoelectric substrate 2 made of a piezoelectric substance such as LiNbO3 or LiTaO3, on which there are provided reflectors 4 and InterDigital Transducers (IDTs) 12, 14 and 16 arranged in directions of SAW propagation. The reflectors 4 are provided at opposite sides of a set of IDTs 12, 14 and 16, and reflect the surface acoustic waves to confine the surface acoustic waves in the set of IDTs 12, 14 and 16. Each of the IDTs 12, 14 and 16 is composed of a pair of comb electrodes that are interleaved. The IDT 14 has a pair of comb electrodes 14a and 14b. The comb electrode 14b of the IDT 14 is connected to an unbalanced terminal 6, and the comb electrode 14a thereof is connected to a ground terminal 3. The IDT 12 and IDT 16 are 180 degrees out of phase, and are composed of pairs of comb electrodes 12a and 12b and 16a and 16b, respectively. The comb electrodes 12a and 16a of the IDTs 12 and 16 are connected to balanced terminals 8a and 8b, respectively. The comb electrodes 12b and 16b of the IDTs 12 and 16 are connected to ground terminals 3, respectively. An unbalanced signal that is input via the unbalanced terminal 6 is output via the balanced terminals 8a and 8b as balanced signals. That is, the SAW device illustrated in FIG. 1A functions as a balun.
As illustrated in FIG. 1B, IDTs 38, 40 and 42 are provided in the directions of SAW propagation. The unbalanced terminal 6 is connected to the IDTs 38 and 42. One of a pair of comb electrodes that form the IDT 40 is divided into comb electrodes 40a and 40b, which are respectively connected to the balanced terminals 8a and 8b. The SAW device equipped with the IDT 40 having the divided comb electrodes has the balun function.
The antenna and the amplifier connected to the SAW device may have different impedances. It is thus required that the SAW device has not only the balun function but also an impedance converting function between the unbalanced terminal side and the balanced terminal side.
Japanese Laid-Open Patent Publication No. 10-117123 (Document 1) discloses a device equipped with a filter formed by cascading SAW filters that are 180 degrees out of phase and a filter formed by cascading SAW filters that are in phase. An unbalanced terminal is connected in parallel with the two filters, and balanced terminals are respectively connected to the two filters. With this structure, the output impedance is four times that of the input impedance and the balanced operation can be realized. Japanese Laid-Open Patent Publication No. 2006-229487 (Document 2) discloses an arrangement in which an unbalanced terminal is connected to four IDTs connected in parallel and balanced terminals are connected to divided IDTs. With this arrangement, the impedance ratio of the impedance of the unbalanced terminal side and that of the balanced terminal side is equal to 1:16. Japanese Laid-Open Patent Publication No. 2006-237727 (Document 3) discloses an arrangement in which an unbalanced terminal is connected to two filters in parallel, and each of two IDTs to which balanced terminals are connected are connected in series to other IDTs. With this arrangement, the impedance ratio of the impedance of the unbalanced terminal side and that of the balanced terminal side is equal to 1:16.
However, the arrangements disclosed in Documents 1 through 3 realize only limited impedance conversion ratios, namely, 1:4 and 1:16.